John Gramlich of Pew Research –
The United States holds a presidential election every four years, but it’s not just the candidates and issues that change from one campaign cycle to the next. The electorate itself is in a slow but constant state of flux, too.
The profile of the U.S. electorate can change for a variety of reasons. Consider the millions of Americans who have turned 18 and can vote for president for the first time this year, the immigrants who have become naturalized citizens and can cast ballots of their own, or the longer-term shifts in the country’s racial and ethnic makeup. These and other factors ensure that no two presidential electorates look exactly the same.